piston broke!

[Austin Branson]

Good evening, Oh Sage Ones.

Chasing down a long standing mis-fire, and I stumbled across a complete lack of compression on No.6. Got the offending rod and piston out, middle ring is completely broken up, and so is the land of the piston between the rings. End result- I need a +030 piston . Anyone got one, by any chance?

Thanks,

Austin

[Peter Cobbold]

Good evening, Oh Sage Ones.

Chasing down a long standing mis-fire, and I stumbled across a complete lack of compression on No.6. Got the offending rod and piston out, middle ring is completely broken up, and so is the land of the piston between the rings. End result- I need a +030 piston . Anyone got one, by any chance?

Thanks,

Austin

Hi Austin, Was the land cracked vertically in several places? If so it points to not enough ring gap to take the extra heat from the boost. Good idea to open up the ring gaps on all pistons. Number 6 runs hottest.......you may be lucky the others will then be OK.

Peter

[astontr6]

Hi Austin, Was the land cracked vertically in several places? If so it points to not enough ring gap to take the extra heat from the boost. Good idea to open up the ring gaps on all pistons. Number 6 runs hottest.......you may be lucky the others will then be OK.

Peter

Poor ring gaps is not an uncommon fault with County Pistons, I was told to pay particular attention to this when my engine was re-assembled after a re-bore.

 

Bruce.

[Peter Cobbold]

Poor ring gaps is not an uncommon fault with County Pistons, I was told to pay particular attention to this when my engine was re-assembled after a re-bore.

 

Bruce.

Bruce,

Oh dear. That's even more of a worry with a supercharger. Here's one of 4 that failed together during a full-boost, full rpm, brief uphill blast.

http://prntscr.com/hhh5ff

Not mine...thanks for the photos John.

Peter

[North London Mike]

Moby (our Rally 2.7 Mk2) suffered a similar fate on county pistons, admittedly after 20,000+ very hard miles.

 

#4 down to zero and endoscope examination revealed a broken up piston

 

Currently awaiting Dale to strip and rescue

[Peter Cobbold]

@ Austin,

Aim for 6 thou gap per inch of bore.

from the bible: A Graham Bell " Forced induction performance tuning"

Peter

[astontr6]

@ Austin,

Aim for 6 thou gap per inch of bore.

from the bible: A Graham Bell " Forced induction performance tuning"

Peter

Peter I was told that the ring gaps s/b 0 .012 to 0.015" and the corners to have a radius of 0.005 to 0.010"

 

Bruce.

[Motorsport Mickey]

Bruce,

 

Piston ring gaps are amongst the least difficult areas to arrange, why ? because the actual measurements are so broad as to be easily achieved, the most important measurement of those you gave are the radius on the piston ring corners ! As long as the gaps are not too tight being the only proviso, too big ? can hardly be considered. Read the attached doc which I came across years ago and posted on this forum about 3 years ago.

 

http://www.diagnosticengineers.org/journal_%20articles/Ring%20Gaps%20vs%20Knowledge%20Gaps.php

 

Bottom line is even the piston ring manufacturers agree that compression lost through larger gaps (and we are talking about passing a rabbit wearing a top hat though here !) make buqqer all difference to the power and performance, this is confirmed in the article. Peters 6 thou per inch of bore is a good rule and falls within the realms of where they need to be, ...me ? I just slap a 15thou feeler through the gap as being the minimum measure and don't worry if it's sloppy it's all to the good.

 

Mick Richards

Edited December 1, 2017 by Motorsport Mickey

[astontr6]

Bruce,

 

Piston ring gaps are amongst the least difficult areas to arrange, why ? because the actual measurements are so broad as to be easily achieved, the most important measurement of those you gave are the radius on the piston ring corners ! As long as the gaps are not too tight being the only proviso, too big ? can hardly be considered. Read the attached doc which I came across years ago and posted on this forum about 3 years ago.

 

http://www.diagnosticengineers.org/journal_%20articles/Ring%20Gaps%20vs%20Knowledge%20Gaps.php

 

Bottom line is even the piston ring manufacturers agree that compression lost through larger gaps (and we are talking about passing a rabbit wearing a top hat though here !) make buqqer all difference to the power and performance, this is confirmed in the article. Peters 6 thou per inch of bore is a good rule and falls within the realms of where they need to be, ...me ? I just slap a 15thou feeler through the gap as being the minimum measure and don't worry if it's sloppy it's all to the good.

 

Mick Richards

Mick Whilst I don't doubt the test results, but it does not say what the wear is if in any of the bores or pistons if any? I suspect newly board bores and pistons. But are you going to get the same results on a used engine? For me that is the key question and answer!

 

Bruce.

[RogerH]

Hi Austin,

when I read the title I thought is was a statement of your personal situation at present

 

Roger

[Fireman049]

I well remember coming out of our local pub once ~ I was piston broke!!

 

Tom.

Edited December 1, 2017 by Fireman049

[Peter Cobbold]

Supercharged engines need more gap as their heat throughput at a given rpm can be 50% up on a normal engine. Austin has an early version of the Moss blower kit, and will be running hotter than stock under full boost.

 

The rings are responsible for transfering to the cylinder wall about 70% of the heat that gets into the piston from combustion. So they expand appreciably, and more the hotter the piston runs. And the gap shrinks compared with a cold measurement. I think Austin needs to aim to get around 18thou, although it will be interesting to know what gap the surviving rings have.

 

The ring-land gets hammered by a siezing ring at TDC and BDC. Once the gap has shrunk to zero and beyond.....smoke beckons. At tdc and bdc the expanded ring stops moving, wedged against the wall, and the piston land has to jolt it into motion again. Hence the cracking in Johns photo, the result of the siezed ring bending the land up and down. Bore wear is minimal at tdc and bdc so is only importnat for blow-by/emmisions.

 

Peter

Edited December 1, 2017 by Peter Cobbold

[Motorsport Mickey]

Mick Whilst I don't doubt the test results, but it does not say what the wear is if in any of the bores or pistons if any? I suspect newly board bores and pistons. But are you going to get the same results on a used engine? For me that is the key question and answer!

 

Bruce.

Hi Bruce,

 

It doesn't state that it's a new engine but does state

 

"It is important to stress at this point that we were dealing with compression rings that were brand new when fitted to the test engine" because it's the piston ring end gap which is being evaluated.

Yes you would get the same results on a used engine as regards the effect that increased piston ring gap would generate.

"Practical tests established that the gap was not the villain of the peace. To all practical purposes the size of the gap didn’t matter."

 

"As the periphery wears in contact with the bore, the radial (piston ring) thickness obviously decreases, as does the tangential load. Peripheral wear means a smaller ring o/d and this manifests itself as an increase in the ring gap."

 

The evaluation shows that excess piston ring end gap where generated by wear in the bores has very little effect upon compression and oil usage, and the conclusion is the excess wear in the bores however does remove the piston ring tangential loading (the pressure of the rings on the bore walls) and that is the culprit of excessive oil usage.

 

"It's not the gap but the reduced tangential load that is detrimental to the performance of the engine. The ring gap is a complete red herring. "

 

Mick Richards

Edited December 1, 2017 by Motorsport Mickey